My friend (who used to be my crush) and I had an EE (Extended Essay) topic to discuss with our supervisors, after I introduced mine, my friend introduced his and his topic is related to a conflict I once had with him. It gave me bad memories again and it really ruined my mood for a few days

His EE topic was about protest, back then our FOA topic was the same, we had a fight during the FOA because of me taking things to personally, clearly I was on the wrong and I apologized, he forgave me very quickly and still lets me be his friend. Also I confessed to him and after he rejected for good reasons, I was relieved because of not having to hide anymore secrets.

But the thing is, back to the EE consultation, that time when he introduces his topic, it really made me felt uneasy because it jogged my memory, even his research documents were the same from our FOA. It turned out that I still can't forgive myself even after 4 months, even though he forgave me, even though it happened a long time ago, it didn't make it any less wrong, honestly.

Look, I know I shouldn't take it too seriously or emotionally, but I can't get over it, I honestly don't know why this is very irritating, this has never happened before. Should I distract myself or just tell him, if I really don't want that event to ruin my mood again?

You get over it. It's your problem, not his. Why would you tell him?

How EXACTLY can I get over it? That's what I'm asking

Crushes are a normal and in some ways vital part of youth. They stop happening somewhere near the end of the teens.
Taking things personally, however, is a trait that may never stop, and it's destructive for both you and the other person.
Wanting to draw him back in to your angst again, even after all the apologies and forgiveness, is sort of a subset of that whole mindset. You are much too self-absorbed.
Keep busy!
I think many or most teens are too self-absorbed. I know I was. I didn't even notice that one classmate's mother killer herself, another mother died, siblings died, grief much worse than my home life. Even my best friend had troubles I didn't know about.
Do you know your close friends really well?

[EDIT: I'm guessing that you don't have close friends, from reading older posts.
I wrote this exactly 3 months ago: "Another key phrase to remember, besides 'don't take things personally,' is 'get outside of yourself.' Think about OTHER people more. Compliment them. Ask them how they are, or what they like, or what they thought about a class, or what it was like when they were small... anything about them rather than you. You will be amazed at the results.
Good luck."
You do seem to be learning and getting somewhere. It is a struggle - all teens struggle. Again - good luck. I have a feeling that you'll get through this.]

Tell him nothing as this is your challenge to overcome, and the best thing is to talk to a best friend, or get busy with the other parts of your life that are much more important. You are making this and the feelings you have over it a bigger deal than you should. Don't dwell on the past, live in the present, and be better for the learning experience.